Schottky barrier gate field-effect transistors (SBFET's) fabricated in gallium arsenide are generally well known in the microwave industry and have proven useful as high frequency amplifiers at frequencies up to 12 GHz or higher. These gallium arsenide metal-semiconductor devices are also known in the art as MESFETS, an acronym for Metal Epitaxial Semiconductor Field Effect Transistor, and have been recently described, for example, by D. R. Chen et al, Microwave Journal, International Edition, Vol. 18, No. 11, November, 1975 at page 60, and by Richard T. Davis in Microwaves, Vol. 14, No. 11, November, 1975, at page 38, and also by Stacy V. Bearse in a state-of-the-art type article entitled "GaAs FETs: Device Designers Solving Reliability Problems" Microwaves, February, 1976 at page 32. These gallium arsenide (GaAs) Schottky gate field-effect devices operate at generally higher frequencies than their silicon counterparts as a result of the higher carrier mobilities of gallium arsenide, and they generally include some preconfigured channel region, such as an epitaxial or ion-implanted GaAs channel layer, formed on or in a high resistivity semi-insulating GaAs substrate. Source and drain electrode contacts and an intermediate Schottky gate electrode contact may then be deposited directly on the upper surface of the GaAs channel region. When a suitable control voltage, V.sub.g, is applied between the source and gate electrodes, the width of the channel depletion region beneath the gate electrode can be controlled to thereby modulate the channel conductivity between source and drain electrodes. This Schottky gate field transistor operation is well known to those skilled in the art.
For this type of semiconductor device, it is also well known that the source-drain metallization on the one hand and the gate metallization on the other hand require different metals and corresponding different metallization processing considerations. The source and drain contacts are ohmic contacts and therefore require a low resistance metal, such as a germanium-gold alloy or a germanium-gold-nickel alloy. On the other hand, the Schottky barrier beneath the Schottky gate metallization is best provided by other metals, such as aluminum, which are especially well suited for this particular type if electrical contact. Therefore, the different contact metallization systems used for the source-drain electrodes and gate electrodes, respectively, require corresponding separate and different successive masking procedures which are necessary to define the exact geometries of these device electrodes.